1. Field of the Invention
The present invention is directed to brake testing apparatus and particularly to a roller type brake testing apparatus.
2. Description of the Prior Art
Roller type brake testers conventionally include two aligned pairs of cradling rollers mounted in a horizontal plane for supporting each wheel of a pair of front or rear vehicle wheels. A separate prime mover such as an electric motor is connected to one roller of each pair of rollers for driving the pair of wheels supported on the cradling rollers at various speeds. When the vehicle wheel brakes are actuated to apply braking force, the value of the braking force is conventionally measured by measuring the torque applied by the driving motors.
The driving motors are conventionally connected to the roller of each pair which engages the rear portion of the wheel. As the wheel brakes are actuated the reaction force between the wheel surface and the driving roller is in a rearward direction which tends to force the vehicle off of the rear of driving rollers. The angle between a horizontal plane and a tangent to the central contact area between the wheels and the rear rollers is sometimes referred to as the pull out angle. The greater the pull out angle, the greater the torque that can be applied to the vehicle wheel while maintaining the vehicle wheel positioned on the cradling rolls.
The pull out angle and hence the torque that can be applied to the wheels under test can be increased by increasing the spacing between the cradling rollers or by raising the rear cradling roller relative to the front cradling roller.
The size of the vehicle wheels to be tested limits the maximum spacing between the front and rear rollers. As the spacing between the front and rear rollers is increased the static force between the rollers and the wheels increase for any given size of wheels. Thus, the spacing between the rollers is a compromise between the desired pull out angle and the sizes of the wheels to be tested. The spacing is normally fixed for any given brake tester.
To provide a larger pull out angle for a given roller spacing, prior art devices have been constructed with the rear cradling roller mounted at a higher elevation than the front cradling roller. Such a configuration, however, makes it difficult to position the wheels and particularly the front wheels of a vehicle on the cradling rollers because the wheels must be forced up and over the raised rollers. It is also difficult to remove the wheels from such a brake tester where the installation requires that the vehicle be removed by backing out of the tester. Under vehicle clearances are frequently small, particularly between the front and rear axle. Thus, when the vehicle is moved across the elevated roller type tester interference may occur between the tester and the vehicle. To overcome this problem the elevation of the rollers must be minimal, thus compromising the pull out angle.
The present invention overcomes many of the shortcomings of the prior art brake testers by providing a small pull out angle when the vehicle wheels are to be positioned on the cradling rollers and a large pull out angle when the wheel brakes are to be tested to permit (1) the vehicle to be readily driven onto and off of the rollers, and (2) a high torque to be applied to the wheels under test.